1. Field of the Invention
This invention relates to computer systems, and more particularly to an apparatus and method for monitoring the environment of remote components attached to a central processor by means of a standard interface bus.
2. Description of Related Art
Modern computer systems often use a standard interface bus, such as the well-known Small Computer System Interface ("SCSI") bus, to link a processor unit to a variety of devices, especially storage devices such as optical and magnetic disk drives, and tape drives. Very often, the storage devices are housed in an enclosure that is physically remote from the enclosure housing the central processor. One example is a large database computer system that has many storage units coupled to a single processor unit. In such a case, it is impractical to package the storage units and the host processor in one physical enclosure. Another example of a remote connection between a host and storage devices are systems that locate storage devices in a secured environment, away from the host processor and users, for increased physical security of the information within the storage devices.
Although such storage devices are located remotely from the host processor, it is still desirable to monitor the operating environment of the storage devices, particularly in fault-tolerant computing applications. For example, it is important to know the ambient temperature of a storage device enclosure and the operational status of supporting components, such as the power supply and cooling fans. In a storage device "farm" or array environment, where there may be tens or hundreds of storage devices, it is desirable to provide an indicator (such as a signal light or audio alarm) to identify a failed device to maintenance personnel. Environment sensing circuitry is known in the art. One example is set forth in co-pending U.S. patent application Ser. No. 07/806,603 entitled "ENVIRONMENT SENSING/CONTROL CIRCUIT", assigned to the assignee of the present invention.
Since the environment functions described above relate to the operating condition of a group of storage devices, such information is not generally available from commercial storage devices, particularly those using a standard interface bus. In particular, such information is not available from commercial SCSI-based storage devices. Therefore, an environment monitoring system is needed in the enclosures of the remote, standard bus storage devices to monitor the local environment, and to communicate status information about the environment back to a host processor.
One possible way of accomplishing this function is to add a maintenance bus between the host processor and the environment monitoring system. For example, the well-known RS-232 serial bus interface could be used to connect a host processor to an environment monitoring system. Environment information, such as out-of-bounds temperature status and power status, would be sent from the environment monitoring system to the host over the maintenance bus. One disadvantage of this solution is the cost of adding a specialized maintenance bus to an otherwise standard computer system. The overhead of an extra maintenance bus becomes significant when the computer system includes tens or hundreds of storage devices.
Another possible solution is to treat the environment monitoring system as a standard bus device, and to communicate environment information between the environment monitoring system and the host processor using the protocol of the standard bus. Thus, no extra maintenance bus need be added to the system. This solution is attractive for small database applications in which only a few standard-bus storage devices are needed. However, this solution requires that the environment monitoring system take up one address port on the standard bus. When a standard bus having a limited number of address ports (e.g., the SCSI bus)is used, this solution presents an overhead problem. For example, there are only 8 address ports per standard SCSI bus. In fault-tolerant computing environments, there would be 2 host processors, leaving only 6 device addresses available. Assigning 1 of the 6 SCSI device addresses to an environment monitoring system incurs a 16% overhead. This overhead becomes significant in large database applications with tens or hundreds of SCSI devices. For SCSI-bus systems using the new "wide SCSI" buses, 16 addresses are allowed. The address overhead in such systems is thus 1 of 14, or about 7%. The 7% address-port overhead imposed by an environment monitoring system might be acceptable when the wide SCSI bus is needed for performance reasons, rather than just for environment monitoring reasons. However, at present, very few storage devices use the wide SCSI bus. Consequently, this latter solution is more theoretical than practical at present.
Therefore, there is a need for providing an environment monitoring system that can communicate with a host processor without requiring a dedicated address port on a bus having a limited number of address ports. The present invention provides an apparatus and method for meeting this need.